Light emitting diodes are widely used in consumer and commercial applications. As is well known to those having skill in the art, a light emitting diode generally includes a diode region on a microelectronic substrate. The microelectronic substrate may comprise, for example, gallium arsenide, gallium phosphide, alloys thereof, silicon carbide and/or sapphire. Continued developments in LEDs have resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, may enable a variety of new display applications, and may place LEDs in a position to compete with the well entrenched incandescent and fluorescent lamps.
One measure of efficiency of LEDs is the cost per lumen. The cost per lumen for an LED may be a function of the manufacturing cost per LED chip, the internal quantum efficiency of the LED material and the ability to couple or extract the generated light out of the device. An overview of efficiency issues may be found in the textbook entitled High Brightness Light Emitting Diodes to Stringfellow et al., Academic Press, 1997, and particularly Chapter 2, entitled Overview of Device Issues in High-Brightness Light Emitting Diodes, to Craford, at pp. 47-63.
Much development interest and commercial activity recently has focused on LEDs that are fabricated in or on wide bandgap materials such as silicon carbide, because these LEDs can emit radiation in the blue/green portions of the visible spectrum. See, for example, U.S. Pat. No. 5,416,342 to Edmond et al., entitled Blue Light-Emitting Diode With High External Quantum Efficiency, assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein. There also has been much interest in LEDs that include gallium nitride-based diode regions on silicon carbide substrates, because these devices also may emit light with high efficiency. See, for example, U.S. Pat. No. 6,177,688 to Linthicum et al., entitled Pendeoepitaxial Gallium Nitride Semiconductor Layers On Silicon Carbide Substrates, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
While efforts to date have provided commercially viable wide bandgap semiconductor devices such as those described above, such devices may still benefit from improvements in efficiency. For example, one issue that may reduce the efficiency of a wide bandgap semiconductor device is related to the forward voltages of such devices. In vertical devices, high resistivity layers may result when the thermal energy (kT) is small compared to the ionization energy of the dopants. Such high resistivity layers may increase the forward voltage drop across the device. Thus, reductions in the resistivity of layers may reduce the forward voltages of wide bandgap semiconductor devices and, thereby, improve the efficiency of such devices.
One proposed technique for enhancing device performance is through the use of a short-period superlattice. Such a technique is described in Saxler et al., “Aluminum gallium nitride short-period superlattice doped with magnesium,” Applied Physics Letters, Vol. 74, No. 14, April, 1999, pp. 2023-2025 and Kozodoy et al., “Polarization-enhanced Mg doping of AlGaN/GaN superlattices,” Applied Physics Letters, Vol. 75, No. 15, October 1999, pp. 2444-2446. Such superlattice structures utilize alternating continuous layers of materials in an effort to improve acceptor ionization percentage and conductivity and, thereby, reduce the overall resistivity of the structure.